The ovarian steroid hormones, estrogen and progesterone, promote accumulation of genetic changes during the development of breast cancers, and drive the growth of established breast cancer. During cancer progression, breast tumors become steroid hormone resistant and concurrent amplification of type I (EGFR/c- erbB2) tyrosine kinase growth factor receptors (GFRs) occurs with high frequency;these alterations are associated with a poor prognosis. GFR signalling initiates the activation of cytoplasmic protein kinases, including the mitogen-activated protein kinase (MARK) cascade. MAPKs phosphorylate transcription factors that control the expression of genes involved in the regulation of cell growth and survival. In breast cancer cells, EGFR/c-erbB2 receptors are regulated by steroid hormones, and cross-talk between these pathways is well-established. Steroid hormones, via cognate plasma membrane-associated receptors, also rapidly activate cytoplasmic kinases typically activated downstream of GFRs. We propose that rapidly activated protein kinases regulate the actions of nuclear steroid receptors;rapid activation of MAPKs by progesterone receptors (PR) provides a mechanism for feed-forward regulation of PR transcriptional activity. Phosphorylation of PRs in response to MAPK activation mediates nuclear association of unliganded PRs and greatly potentiates PR transcriptional activity in the absence and presence of progestins. Paradoxically, these events are also required for nuclear export and rapid downregulation of liganded PRs by the ubiquitin- proteasome pathway. We hypothesize that phosphorylation of PR serves to coordinate and couple multiple aspects of PR action, including nuclear-cytoplasmic shuttling, transcriptional activity, and protein turnover. We postulate that these events are linked by common cellular machinery regulated by phosphorylation. This proposal will address the following questions: 1) What is the functional significance of MAPK/MEK interaction with PRs? 2) What is the role of cell cycle mediators in PR action? And 3) How does phosphorylation couple PR shuttling and transcriptional activity to turnover by ubiquitination? These aims will define the molecular mechanisms of PR regulation by key mitogenic kinases (MAPKs and cyclin-dependent protein kinase-2) in breast cancer cells. Growth factors drive breast cancer cell growth by altering steroid hormone receptor function. Progestins contribute to breast cancer risk, in part via cross-talk with MAPK-dependent pathways. Our long-term goal is to define the role of signaling events in the progression of breast cancer. If we can understand the mechanisms that control the switch from growth stimulation by steroid hormones to growth factors, this pathway could, in theory be blocked, leading to longer disease-free survival of patients with breast cancer. This work will aid our understanding of steroid hormone action, and help to identify and implicate new therapeutic targets for breast cancer prevention and treatment.